Made the mistake to leave (and use) the iPod for years on a Bose player/speaker system - which means that in use or not the battery was continuously in a charging state. Recently I noticed the display having two dark spots about 1/4" dia. each. It was from the battery was now swollen out of shape.
I had a hard time to slowly push everything out of the case (because of the swollen battery) but with the help of the "ifixit" guide, a new display and battery, now I have a perfectly working iPod.
Never leave the battery under continuous charge !
Trust the "ifixit" guides on "how to" - without them one would not be able to do it.